The method of retrieving data from a magnetic coated hard disc and putting new data on the disc has certain parallels to the way a record player functions. Like the record played on a record player, the disc rotates at relatively high speed about a central axis. An arm, like the tone arm of the record player, is positioned above the spinning disc to retrieve data off the disc. Unlike the record player, this arm can also put new data on the disc as well as retrieve it.
The arm is called a head gimbal assembly and consists of three components: the load arm, the gimbal and the read/write head. Additionally, connecting leads (not shown) are attached to the read/write head and are routed along the top of the load arm. These leads are retained in place typically by small clips formed in the upper side of the load arm. The load arm is a suspension device usually constructed of metal that is approximately one inch long. The load arm both supports the read/write head and applies a downward force on the read/write head. The gimbal is a moveable attaching device that attaches the read/write head to the load arm. The gimbal permits the read/write head to move in both pitch and in roll as the read/write head moves across the disc.
When viewed end on, the read/write head has two spaced apart lands separated by a groove. When positioned on the load arm, the two lands are oriented tangential to a circle having its origin at the center of the disc. The inner land is located radially nearer to the center of the disc than the outer land. The active portion of the read/write head that actually does the data retrieval and imposition of new data on the disc is located is formed integral to the outer land on the surface of the land that is closest to the disc. This surface is called an air bearing surface.
When the disc is not in operation, the read/write head is resting on the disc in response to the force imposed by the load arm. When the disc is brought into rotating motion , the read/write head is lifted off the surface of the disc by a stream of air known as an air bearing. The force of the air bearing counters the force of the load arm and the read/write head actually flies over the surface of the disc and is subject to many of the same aerodynamic forces that an aircraft is when in flight. For accurate data retrieval and imposition, it is very important that the read/write head flies very accurately over the disc. Precisely locating the read/write head with respect to the gimbal at the time that the read/write head is adhesively bonded to the gimbal is a major factor in ensuring the accurate flight characteristics of the read/write head.
There is a present problem of properly locating and adhesive bonding the Load Arm Gimbal to the read/write head. At present, flying heights between the air bearing surfaces of the read/write head and the magnetic disc surface are approaching 5.mu. inch spacing. Manufacturing tolerances in the Load Arm Gimbal assembly and lack of optimum positioning of the read/write head cause unwanted errors in flying height of finished HGA assemblies. This greatly diminishes the yield rate of finished HGA's. Additionally, since the surface velocity over the rotating disc at a given radius of the disc is greater than the surface velocity over the disc at a lesser radius, the outer land of the read/write head sees a greater surface velocity than the inner land. This greater speed increases the aerodynamic lift on the underside of outer land, known as an air bearing surface (ABS). This greater speed caused the outer ABS to fly higher above the disc surface than the inner ABS as a result of an increased aerodynamic lift.
As previously indicated, the active read/write magnetic transducer is located on the outer air bearing surface of the read/write head. This is desirable in order to use the largest disc surface available for read/write recording and to optimize the amount of recorded data on the disc. It is also desirable to have the active transducer, and thus its supporting ABS, fly at a height equal to or lower than the inactive inner ABS. Offsetting the gimbal toward the outer ABS prior to bonding the read/write head to the gimbal will cause the outer ABS to fly lower and the inner ABS to fly higher. This must be very accurately done to achieve the desired result.
Another present source of error in the production of HGA's is the fact that the underside surface of the load arm to which the gimbal is attached may not be exactly coplanar with the upper surface of the read/write head where the read/write head is bonded to the gimbal. Manufacturing tolerances on coplanarity between the load arm mounting surface and the gimbal to read/write head interface surface are currently 0.5.degree. to 1.degree.. While such angular difference is relatively small, this tolerance causes additional variation of differential flying height between the two air bearing surfaces.
Present industry convention defines positive direction differential flying height or roll as the outer ABS flying lower than the inner ABS. To ensure the proper flight height and roll, each ABS must be instrumented to measure the vertical load and the load on each ABS will necessarily be different at the time that the read/write head is bonded to the gimbal. The measuring devices must be able to accurately measure very small loads on the order of 2.0 gm, to an accuracy and resolution of 50 milligrams.
The current method of instrumenting the ABS loads is through the use of a load cell that measures only tile total downward force on both ABS taken together. No current system measures the differential gram loading on each ABS. Effectively the current method of positioning the head with respect to the load arms to determine that on the average, the head should be, for example, 2.5 mils right of center on the load arm. The current systems are capable of very accurately positioning the head in that position on the load arm. The problem is that the position is only an estimate of where the head should actually be positioned in order to provide the optimum performance.
A further disadvantage of load cell systems stems from the deflection of a load cell under load, especially when a differential load on each ABS is required for accurate positioning. The load cells tend to form a plane which is at an angle with the load arm mounting base and thereby introduce an angular error which will reduce the accuracy of the measurement.
Another disadvantage of load cell use occurs from the inability of the load cells to measure an overturning moment. An overturning moment occurs due to unrelaxed friction between the gimbal and the upper surface of the read/write head that results from moving the read/write to the desired position beneath the gimbal and from torquing moments applied to the load arm at its point of mounting in the head gimbal locator device. A side force due to an unrelaxed frictional force and the torquing moments will result in an overturning moment. The side force acts on the read/write head in such a manner as to attempt to rotate the read/write head in either a clockwise or counterclockwise direction. This rotational force will be measured as an increase in the downward force on one ABS and decrease in the downward force on the other ABS depending on whether the rotational force is operating in a clockwise or a counterclockwise direction.
As indicated, the current devices used for measuring the force on the two air bearing surfaces of the read/write head read only the total gram loading as distinct from the present invention that reads both the total gram loading and the differential gram loading on each ABS. The current devices and are therefor prone to error. This error is necessarily reflected in the positioning of the read/write head with respect to the gimbal prior to bonding and results in ultimately mispositioning the read/write head on the gimbal and thereby causing poor flight characteristics of the read/write head.
The read/write head is a very small device and, in keeping with the trends in the electronics industry, successive generations of read/write heads keep getting smaller. This trend intensifies the need to more accurately position the head on the load arm. Accordingly, it would be a decided advantage in the disc file industry to have a head gimbal locator that would accurately measure differential loads on the two air bearing surfaces of the read/write head and accurately position the read/write head with respect to the load arm and gimbal to achieve a desired differential loading of the ABS's in order to ensure the desired flight height of the read/write head when the read/write head is in operation with a rotating disc.